1. Field of the Invention
The present invention concerns an electro-optic modulation method and device. The present invention may be used in optical telecommunications applications, such as for high flow numerical linkages over a long distance, as well as in electro-optic switching applications.
2. Discussion of the Background
The present invention forms a part of techniques for modulating the intensity of a luminous beam and which makes use of the phenomenon of electro-absorption. This phenomenon, which makes its appearance in a semiconductive material, concerns the optical absorption variations of a material under the effect of an electric field.
An electric field is applied to a semiconductor material, for example, with the aid of a PIN type diode 2, as shown in FIG. 1, or Schottky type diode which is polarized by a suitable device 4 and in which the intrinsic region I (not deliberately doped) contains the electro-absorbent material.
Certain techniques use a "perpendicular" type configuration, shown in FIG. 1-A, in which the light is sent onto the diode along the growth axis Z of the diode. Other techniques use a "guided waves" type configuration, shown in FIG. 1-B, in which the light is injected into the intrinsic region I parallel to the plane of the films P, I and N.
Compared with the "perpendicular" type configuration, this "guided waves" type configuration makes it possible to increase the interaction length on which the light is absorbed by the semiconductive material, namely the major part of the guided light situated in the region I.
The various techniques for modulating the intensity of a luminous beam, which use a "guided waves" type configuration, are differentiated by the physical effect implemented so as to obtain the optical absorption variation under the effect of an electric field.
Up until now, three physical effects have been proposed: the Franz-Keldysh effect, the Stark effect confined quantally and the "blue shift" effect of the absorption threshold in tightly coupled super-grid or super lattices.
The Franz-Keldysh effect is a "red shift" of the absorption threshold of a compact semiconductive material in the presence of an electric field.
FIG. 2-A shows a valence band BV and a conduction band BC of the semiconductive material which are separated by a forbidden band with a width Eg in the absence of any electric field (E=0).
FIG. 2-B shows that the application of an electric field to the semiconductive material (E.noteq.0) breaks the periodicity of the material in the direction of this field, a periodicity which created the forbidden band.
This rupture of periodicity makes it possible to have electrons and holes in the "ex-forbidden" band in which the probability density of the presence of the electrons De and the probability density of the presence of the holes Dt are not nil.
There then appears a transition possibility at an energy EfK, which is less than Eg, which corresponds to a red shift of the absorption threshold.
This is diagrammatically illustrated by FIG. 3 which shows the absorption spectrum of a compact semiconductive material (curve of the variations of optical absorption a according to the wavelength l) in the presence of an electric field (E.noteq.0) and in the absence of any electric field (E=0).
This red shift makes it possible to obtain absorption variations da (usually about 100 cm.sup.-1 for an electric voltage of 5 V applied to a thickness of 0.5 micrometers) in the domain of wavelengths where the material is transparent and thus to embody a modulator functioning at the wavelength lo indicated in FIG. 3.
One example of the embodiment of a modulator using the Franz-Keldysh effect in a "guided waves" type configuration is described in an article by Y. Noda et al., published in the IEEE Journal of Lightwave Technology, vol. LT-4, No 10, Oct. 1986, pp. 1445-1553.
The quantally confined Stark effect is a red shift effect of the absorption threshold of a multiple quantum-well structure.
For the purpose of simplification, FIG. 4 shows the band structure of a quantum well in the absence of any electric field (FIG. 4-A) and in the presence of an electric field (FIG. 4-B).
FIG. 4-A shows that in the absence of any electric field, the basic energy level Et of the holes are separated by an energy Ee-t.
In the presence of an electric field (FIG. 4-B), these energy levels in the quantum wells are displaced and the electron-hole transition energy (Ee-t) is reduced by a quantity dE, hence resulting in a red shift of the absorption spectrum.
This is diagrammatically shown by FIG. 5 which shows the absorption spectrum of a multiple quantum-well structure in the presence of an electric field (E.noteq.0) and in the absence of any electric field (E=0).
The absorption spectrum of this multiple quantum-well structure has a "staircase" working rate to which are added the excitonic absorption peaks whose origin is the electron/hole hydrogenoid interaction. The entire spectrum is red shifted by means of the quantally confined Stark effect.
Thus, it is possible to obtain highly significant absorption variations da in the domain of wavelengths where the structure is transparent (typically about 1000 cm.sup.-1 for an electric voltage of 5 V applied to a thickness of 0.5 micrometers).
One example of an embodiment of a modulator using the quantally confined Stark effect in a "guided waves" type configuration is described in an article by K. Wakita et al, published in Electronics Letters, 13 Oct. 1988, vol. 24, No 21, pp. 1324 and 1325.
The blue shift effect of the absorption threshold in tightly coupled super-grids is described in an article by J. Bleuse, G. Bastard and P. Voisin published in Phys. Rev. Lett., vol. 60, No. 3, 18 Jan. 1988, pp. 220-223, in an article by J. Bleuse, P. Voisin, M. Allovon and M. Quillec, published in Applied Physics Letters, 53 (26), 26 Dec. 1988, pp. 2632-2634, and in the French Patent Application No.
Mention is made hereafter of three essential parameters for evaluating the performance of a device for modulating the intensity of a luminous beam. These parameters are the extinction rate, attenuation in the "on" state and the control voltage of the device.
The extinction rate is the ratio of the luminous beam leaving the modulator in the "on" state to the luminous intensity leaving the modulator in the "off" state. This extinction rate may be expressed in percentages or in dBs. It needs to be as high as possible (typically about 20 dBs). It is directly related to the absorption variation.
Attenuation in the "on" stat is the ratio of the luminous intensity leaving the modulator in the "on" state to the luminous intensity entering into the modulator. This attenuation in the "on" state may be expressed in dBs. It needs to be as low as possible (typically less than or equal to about 3 dBs). It is directly related to the residual absorption in the domain of wavelengths where the electro-absorbent material of the modulator is transparent.
The control voltage of the device needs to be as weak as possible. It is directly related to the electric field to be applied to the electro-absorbent material the device contains.
In relation to these three parameters, the techniques which respectively use the three effects mentioned above exhibit the following drawbacks.
The modulators using the Franz-Keldysh effect and those modulators using the quantally confined Stark effect require that high electric fields be applied, usually about 10 V per micrometer, namely a voltage of 5 V applied to a thickness of 0.5 micrometers. Such control voltages result in a high energy dissipation in cases of fast modulation (frequencies of several GHz) .
Modulators using the blue shift effect present significant attenuation in the "on" state (that is, when the electric field is not nil). This is valid for two reasons:
1) the operating point of a modulator of this type is situated on a wavelength extremely close to the absorption threshold; the enlargements linked to the temperature and the shortcomings of the super-grid make an absorption tail appear which is located towards the low energies and whose effects are considerably close to the absorption threshold; PA1 2) at the same time as the blue shift, a low-energy "oblique" transition appears, this being shown in FIG. 8 described subsequently.
This attenuation in the "on" state is extremely considerable, as indicated on page 1551, left column, lines 6 to 11 of the article by R.H. Yan et al, published in Applied Physics Letters, vol. 54 (16), 17 Apr. 1989, pp. 1549 to 1551.